"Magics" support

[l4l]

Currently, apps are launched by their Exec=... entries in desktop files. Sometime it's useful to pass program arguments to the command. Thus "magics" are considered, which is an extension to the search field (e.g. somewhat inspired by dgg's bangs). Here's the sample list:

• x !! y executes a program found by search string x with arguments listed in y;
• x ~ y executes a program found by search string x with working directory y;
• x # y executes a program found by search string x with env vars specified in y (e.g. MYVAR=xyz MYVAR2= MYVAR3=abc);
• @ x executes a binary x as-is without any search;
• ...anything else? (let it be discussed in another issues)