texthistory: hoist single actions from group

texthistory: hoist single actions from group

In many cases across GtkTextBuffer, we end up with operations performed
inside of a begin/end user action. Those can be coalesced into a single
sub-action within the group, and hoisted out of the group. Doing so
increases the chances that we chain similar actions together for words.

Additionally, this fixes an issue introduced in
6179886b14eccd3034dc8121aac7b709aac073ec for #3977 where GNOME Text Editor
started to group all possible actions into a single group.

Fixes GNOME/gnome-text-editor#97