characters: Refactoring, improve validation in kcm, optimize

Summary:
The config keys are now in a separate file, instead of having them at two places.
Runner
The duplicate call to reloadConfiguration has been removed and aliases/codes get validated.
Additionally the match method has been simplified and the character is now copied to the clipboard when selected.
KCM
Validation of the add/delete buttons, error message now in GUI and deprecated methods removed.

Test Plan:
Compile, test runner and test editing entries.
Manually add alias(and no hex value) to ~/.config/krunnerrc and reload the kcm, error message should be shown.

Before:

After:

Reviewers: davidedmundson, ngraham, sitter, broulik, Plasma

Reviewed By: sitter

Subscribers: davidre, dvratil, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27096