Overhaul right drawer's UI

Summary:
The right drawer's UI was nonstandard and violated numerous HIG guidelines. This
patch fixes that by doing the following:

• Move the configuration settings to a new page on the stack
• Make the picture/video directory choosers use a common and comprehensible UI
• Don't ask for confirmation before moving items to the trash
• Disable rather than hide inapplicable actions
• Give the back buttons arrows
• Display an informative message when there are no pictures to show
• Fix ellipsis use
• Use correct casing for titles and buttons
• Improve button title formatting and use i18np() where appropriate
• Port a few deprecated uses of Kirigami.Label to the QQC2 version
• Improve header text margins

Test Plan:
Before:

After:

New message when no Kamoso-taken images are in the current pictures directory:

New Configuration page:

Reviewers: apol, VDG, GB_2

Reviewed By: apol, VDG, GB_2

Subscribers: GB_2

Differential Revision: https://phabricator.kde.org/D18170