PR: Make some UI/UX improvements to the Find pane

[ccordoba12]

Description of Changes

• In SearchInComboBox, disable clear list item and hide separator between regular items and other directories when users have not selected any other directory.
• Remove tooltips from the SearchInComboBox items because they were annoying and unnecessary.
• Change text of the action shown in the Search menu that points to the Find pane. That's to give users a better idea about it.
• Fix bug when clicking on that action and (apparently) Spyder can't grab text for it.
• Use an enum for the index items used in SearchInComboBox. This allows us to access them more easily.
• Leave results browser visible after the first search is performed. This was very annoying because the empty message was shown when a new search was started or when no results were found.

Visual changes

Before

After

Issue(s) Resolved

Fixes #21538

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: @ccordoba12

[pep8speaks]

Hello @ccordoba12! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file spyder/plugins/findinfiles/widgets/tests/test_widgets.py:

Line 740:80: E501 line too long (80 > 79 characters)