widget: Use the same default for word-chars as gnome-terminal

https://bugzilla.gnome.org/show_bug.cgi?id=727743
widget: Remove word-chars property and related API

https://bugzilla.gnome.org/show_bug.cgi?id=727743
widget: Rename API

For consistency.
widget: Add match flags for search regex

For API consistency.
all: Rename API
tests: Fix for API change
widget: Simplify word-chars handling

Instead of having a small set of characters listed explicitly,
just use the character's unicode categories.

Coincidentally fixes https://bugzilla.gnome.org/show_bug.cgi?id=700217 .
widget: Remove unused variable
widget: Add window geometry helper function

Encapsulate this into a helper function that'll be used in gnome-terminal,
instead of having to poke at the internas like character width and height.