config: fix gesture dispatcher parsing with whitespaces (#11784)

* config: fix gesture dispatcher parsing with whitespaces

Some dispatcher functions (e.g., `moveFocusTo`) expect the given string to be
stripped of whitepsaces.

This fixes `gesture` line parsing: rather than calling dispatcher functions
with the original string, we reuse words parsed by `CConstVarList` and join
them with a comma.

* tests/gestures: Add a test for `movecursortocorner`