[quicklaunch] Port settings window to QQC2+Kirigami FormLayout and modernize UI

Summary:

• Port to QQC2+Kirigami FormLayout style
• Use standard import names
• Adjust some strings to sound a bit more natural
• Use Kirigami ActionTextField so that the text field can have a clear button
• Fix a bug preventing the text field from being disabled once enabled

Test Plan:

• All features still work
• Settings are saved and loaded correctly

On the desktop:

In a horizontal panel:

In a vertical panel:

Reviewers: VDG, Plasma, filipf

Reviewed By: VDG, filipf

Subscribers: filipf, plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D21088