change mod/set hardware Search key for OTG mode

[elig0n]

• I have checked that a similar feature request does not already exist.

Describe the bug
Although mod key shortcuts aren't supposed to work in OTG mode, it is still possible to use android's hardware keyboard shortcuts for basic usage of-which most are using the SEARCH button as a modifier for combinations.

For me it appears that that key is the super/win key and while combinations done with it give rise to no problems so far using the key alone (as in a single press) for its Search/Assistant application cannot and does not work because it already serves to detach the mouse-keyboard capture in scrcpy.

A possible solution can be to set either the hardware Search key or alternatively map the detach functionality from Super to something else (I have already tried the --shortcut-mod option unsuccessfully).

[rom1v]

I'm ok for adding a new option --mouse-capture-key=key1,key2,…, similar to --shortcut-mod=… introduced in #1598. However, Super is often captured by the host system, so not all combinations will work.

Would you like to work on it?

FYI, I decided to hardcode the mouse capture key to Alt and Super, but I initially considered using the --shortcut-mod value for the capture key, but this is incorrect for two reasons:

• the --shortcut-mod value might contain a combination of keys (--shortcut-mod=lctrl+lalt+lshift, see shortcuts), while the capture key is only one key (otherwise it would add unnecessary complexity to track press+release actions in all situations, if some keys of the combo is pressed while some other keys are released).
• the --mouse-capture-key might contain keys other than modifiers (for example F12).