PR: Add switcher integration to projects

[AngelaRemolina]

Description of Changes

1. If there is an active project opened, the switcher now loads the files of that project searching them with fzf.
2. Project files can be loaded into the editor from swicher selection
3. When the user types the search text the name of a project in the switcher dialog it is filtered by fzf
4. Added a unit test to assert number of files loaded in the switcher with the editor+projects files.
5. Wait for the user to stop typing (300ms) to execute fzf function (with QTimer) instead of doing it everytime text changes.

Future work:

• Migrate the fzf subprocess function to ProcessWorker

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

Issue(s) Resolved

Fixes #3860

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct:

@angelasofiaremolinagutierrez

[dalthviz]

Thanks @angelasofiaremolinagutierrez for all your work here! Left a couple of suggestions for some docstrings and some comments about the signal to open the switcher selection in the Editor. Also, left some comments/ideas regarding the usage of the switcher reference from the main widget. However, regarding that last part (usage of the switcher reference from the main widget), what do you think @ccordoba12 ?

I'm missing running this changes locally but seems like everything should be working 👍

If you have any comment or question regarding the comments in the review let me know!

[dalthviz]

Checked locally the PR and seems like some logic to handle the case when fzf is not installed needs to be added? I tried without installing fzf first and I got:

Traceback (most recent call last):
  File "e:\acer\documentos\spyder\spyder otros\angela\spyder\spyder\plugins\projects\plugin.py", line 447, in handle_switcher_modes
    self.get_widget().handle_switcher_modes("")
  File "e:\acer\documentos\spyder\spyder otros\angela\spyder\spyder\plugins\projects\widgets\main_widget.py", line 617, in handle_switcher_modes
    paths = self._execute_fzf_subprocess()
  File "e:\acer\documentos\spyder\spyder otros\angela\spyder\spyder\plugins\projects\widgets\main_widget.py", line 732, in _execute_fzf_subprocess
    out = subprocess.check_output(cmd_list, cwd=project_path,
  File "C:\Users\dalth\anaconda3\envs\spyder-dev\lib\subprocess.py", line 424, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "C:\Users\dalth\anaconda3\envs\spyder-dev\lib\subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "C:\Users\dalth\anaconda3\envs\spyder-dev\lib\subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "C:\Users\dalth\anaconda3\envs\spyder-dev\lib\subprocess.py", line 1420, in _execute_child
    hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
FileNotFoundError: [WinError 2] El sistema no puede encontrar el archivo especificado

Left new suggestions that maybe could help with the tests failing and the handling of the case when fzf is not installed.

Testing locally, after installing fzf and applying the suggested changes, things are looking good (a little bit of lag but that is expected 👍):

[ccordoba12]

Thanks @angelasofiaremolinagutierrez for your work on this!

[ccordoba12]

Last stylistic suggestions @angelasofiaremolinagutierrez, then this should be ready on my side.

[ccordoba12]

@angelasofiaremolinagutierrez, thanks for your hard work to implement this feature!! It's a great improvement!