[Patch] Add new split mode for quick splitting of ways

[Woazboat]

Add a new mode for quick splitting of ways by selecting split points via mouse click.
Ambiguous split actions can be resolved by choosing the correct way in a popup dialog or by selecting the targeted way beforehand.
Filters can be used to narrow down which ways are considered for splitting (e.g. filter for highway=* if you only want to split roads)

​https://github.com/JOSM/josm/pull/96
​https://patch-diff.githubusercontent.com/raw/JOSM/josm/pull/96.patch

[Woazboat]

For now only splitting at preexisting nodes is implemented. Adding new nodes into way segments and splitting at those is on the todo list.

I re-used the icon for the split way action for the mode. Maybe this could be replaced with a more illustrative one (e.g. a pair of scissors over a way)

[Woazboat]

This is somewhat similar to the knife tool in the auto-tools plugin (​https://github.com/JOSM/auto-tools#knife-tool), but I think this is useful enough to be implemented in the core

[taylor.smock]

In 18759/josm:

Fix #22279: Add new split mode for quick splitting of ways (patch by Woazboat, modified)

Modifications are as follows:

• Simplify/inline some variables
• Move an inner class used only in a class that it is not in into the appropriate class
• Fix a pre-existing bug where InputEvent.META_MASK would not be mapped to InputEvent.META_DOWN_MASK
• Override some additional methods in SplitMode (layerIsSupported and getModeHelpText)