Revise to be more easy

An the "Welcome to GIMP" section of the introduction, revise two
paragraphs accordings to jmatsuzawa's review since these descriptions
are also added to the GIMP UI translation.
Prepare for translating advanced dialog

Prepare for translating the advanced dialog of Selection to Path.
On GIMP 2.10 development, gettext macro is applied to this very
complicated options UI.