Some fixes and simpifying startup settings.

Summary:
Replaced custom slots for editing key bindings and shortcuts with default slots from KXmlGui

Set status bar visibility more reliable

Cleaning and fixing save/restore of startup settings. WIP

• maximize switch for window state not needed and buggy
• saveMainWindowSettings() saved but never read (wrong config group)
• no danger of initializing default statusbar. Custom one is always set in constructor
• most config options should be independent from "UI save settings"

Cleaning and fixing save/restore of startup settings: Updated settings dialog to reflect changes

Cleaning and fixing save/restore of startup settings: Save UI components on shutdown by default

Test Plan:

• tested save/restore of window position/size in multi-monitor setup
• tested "start to tray" option
• tested "save position" option
• tested with old config file
• tested with no config file (clean start)

Reviewers: Krusader, martinkostolny

Reviewed By: Krusader, martinkostolny

Subscribers: martinkostolny

Differential Revision: https://phabricator.kde.org/D3353