Add relevant tactic snippets to help output

[raulTUe]

Comes as a result of issue #198. The help button currently returns a message saying "hint: replace with ". The goal is to add any relevant tactics along with a button to copy/insert it into the input area.

For instance, an snippet of the help output currently is:
"The goal is to show a 'there exists'-statement (∃). Choose a specific variable in ℝ.
Hint, replace with: Choose ... := ...."

Here, tactics that begin with the keyword Choose could be placed below this text, e.g:
"The goal is to show a 'there exists'-statement (∃). Choose a specific variable in ℝ.
Hint, replace with: Choose ... := ....
Tactics:
Choose (name_var) := (expression)."