simplify and clean keybinding merge code

[vssdeo]

• Current list of Keybindings in Preferences->Keybindings are shown after merging
• The merge happens in prefseditor.py
• Cleaning and moving the code from prefseditor.py
• Adding a function to get plugin via name
• Adding some missing keybindings in config.py and prefseditor.py and syncing them for consistency
• These changes were also part of Feature: is there any way to Hide or Sort context menu items? #773 and pull request [Plugin ContextMenuPlugin] 773-Feature-is-there-any-way-to-Hide-or-Sort-context-menu-i… #842 so decoupling these as separate issue