rework custom stylesheet loading

[deltragon]

Description

Previously, the stylesheet was copied to the user's .config dir, and this was then loaded as a style (with priority "application").
The user could alter this stylesheet directly.
This stylesheet was called "safeeyes_styles.css".

Now, we split the stylesheets into two:

• The system stylesheet, which is directly loaded with priority "application".
• The user's stylesheet, if it exists, which is loaded from the .config dir with priority "user", overriding both the system stylesheet and any themes the user might have.
  This is now called "safeeyes_custom_styles.css".

This means in the future, we can simply add new default styles to the system stylesheet without breaking the user. Additionally, any styles loaded with a higher priority than "application" (eg. certain GTK themes, see #687), can now be overridden by the user's stylesheet.

We also check if the old path ("safeeyes_styles.css") contains an old default stylesheet. If it does, we delete it.
If it (likely) contains a customized stylesheet, don't delete it, but warn the user that they should use "safeeyes_custom_styles.css" instead now.

[archisman-panigrahi]

It fails to run with the following error

python3 -m safeeyes
Traceback (most recent call last):
  File "/home/archisman/SafeEyes-style-priorities/safeeyes/safeeyes.py", line 108, in do_startup
    self._initialize_styles()
  File "/home/archisman/SafeEyes-style-priorities/safeeyes/safeeyes.py", line 169, in _initialize_styles
    utility.load_css_file(
TypeError: load_css_file() missing 1 required positional argument: 'priority'
Traceback (most recent call last):
  File "/home/archisman/SafeEyes-style-priorities/safeeyes/safeeyes.py", line 154, in do_activate
    if self.plugins_manager.needs_retry():
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'needs_retry'

[deltragon]

Thanks, I moved the function around and didn't check it... Should be fixed now.