UX improvements for at-commands

[valaises]

Changelist:

• at-commands are parsed whenever they are: even in the middle of the sentence
• you could place multiple at-commands in a single line
• at-commands do not disappear without a trace -- now they leave a short message
• capabilities for highlighting commands

[olegklimov]

join execute and text_on_clip

@definition "Frog (definition in xxxx.cpp attached)"

@references "Frog"

@workspace "copy of args"

@file fn.ext ""

@symbols-at fn.ext:1337 ""