Revamp Preferences Window

[ochi12]

Progress:

• Revamped Preferences Page to General Page Instead
• port depracted components in Custom Icons Page
• Revamped Custom Icons Page.

Problems havent been resolved:

• auto delete empty cell on edit
• make the column view more visible (like a excel spreadsheet).

finishing task:

• clean up source (scoped by PR only)

The goal of this PR is to utilize the fillPreferencesWindow() of the ExtensionPreferences class. This will allow us to easily to add Pages which are controlled by Adw.ViewSwitcher

New look: INCOMPLETE

Old Look:

[ochi12]

I still havent solve the problem on auto deleting a row.

[ochi12]

@3v1n0