Custom modifiers to give users control over SHIFT/CTRL/ALT keys

When using many tools, Inkscape currently hard codes which modifier keys are used for what kind of functionality.

In order to provide users with some control, this merge request seeks to add functionality that would allow keys.xml to define custom modifier keys.

For example, if you wanted to change Ctrl and Shift around during the resizing process.

Todo list:

• Define basic structures using a sample of modifier keys
• Use modifiers in tool event code
• Allow modifiers to be modified by keys.xml
• Make sure status bar messages match assigned modifiers (modifiers must be able to generate repr string)

Deferred to a different merge request:

• Show modifiers (defaults) in preferences
• Allow keys.xml modifiers to be set in preferences

Notes:

• The option "wheelzooms" which swaped what the ctrl key did for the canvas is REMOVED. Users should instead configure the scroll modifier setting for zoom (and canvas scroll).