configurable shortcuts

[albfan]

Shortcuts may vary from OS to OS.

Allow shortcuts to be redefined, saved and loaded. Add a table to search, filter and redefine.

Work is expected to be done on https://github.com/albfan/jmeld/tree/shorcuts

[albfan]

@KewlCat shortcut for Ctrl+W works well, but Ctrl+Q loops indefinitely when any buffer is modified and unsaved (any cancel should abort operation). squash too last two commits so any commit compiles correctly.

How do you feel about build a shorcut editor? I have made a shallow clone from jedit, to see how to make a shorcuts editor:

• http://xilize.sourceforge.net/Reference/guide/jeditTips.html#shortcuts
• https://github.com/albfan/jEdit/blob/d4a4651320e447ba860ea82d7d26e87bdf6057c2/org/gjt/sp/jedit/options/ShortcutsOptionPane.java