Fix exec quotes

[Itsskiip]

Related issue: #311

Modifications to the get_exec_string function:

• New escape_arg function defined according to the escape/quotation rules at https://specifications.freedesktop.org/desktop-entry-spec/1.1/exec-variables.html
• Replaced sh -c with env for firefox browsers so that the above rules apply correctly to arguments
• Refactoring to facilitate the above change
• Should now be fully compliant except for custom_parameters, which remains unvalidated for now