GUACAMOLE-192: Select word in terminal on double click

[corentin-soriano]

Allows selecting a word on double-click and a complete line on triple-click.

Based on the unmaintained PR by @Saiyeux (#406) :

• All "rewritten" functions have been removed in favor of existing functions.
  -> Code simplification.
  -> Fixes several unexpected behaviors when scrolling upwards.
• The following characters are now considered part of a "word": [0-9\/A-Za-z_\~\&\-\.\?\\%=:$]
  -> Allows selecting an IP address, a path or URL (ex: https://www.test.tld/file.php?param1=value1&param2=value%202)
• Improved condition in the guac_terminal_select_update (select.c) function to allow selecting a "word" of a single character on double-click (no impact on single-click).