Fix meta not being a modifier.

[lzhoucs]

We added super -> meta for GTK in #331, however we didn't convert the modifier state. This has left inconsistency for super key events from GTK which is converted to a regular meta key by us. This causes issues in lapce when super key is used as a modifier key, e.g: when adding a key binding for save by pressing super + s, only s is captured as shown below:
before this fix - press super + s:

after this fix - press super + s:

We can also see the issue in the example app event_viewer:
before this fix - press ctrl key:

before this fix - press super key:

after this fix - press super key:

[jneem]

Thanks, looks good!