Cinnamon keyboard layout switch steals shortcut combinations

 * Cinnamon 3.8.9
 * Mint 19
 * 64 bit

Issue
Setting a keyboard shortcut for switching between keyboard layouts makes them unusable in any other keyboard shortcut. For example, setting the switching shortcut to "Ctrl+Shift" makes you unable to switch to "previous tab" in Firefox (Ctrl+Shift+Tab). This happens because the keyboard switcher "steals" the Ctrl+Shift combination and the target application then receives the Ctrl+Tab event.

Steps to reproduce
Set at least 2 keyboard layouts. Set the "Switching to another layout" shortcut to Ctrl+Shift.
Then open Firefox with multiple tabs and try to use the "Switch to previous tab" shortcut - Ctrl+Shift+Tab. You will see it behaves just like the "Switch to next tab" shortcut - Ctrl+Tab.

Expected behaviour
Cinnamon should not steal the shortcut as soon as it registers the correct keypress. Instead, it should wait for the key release to decide whether to switch the keyboard layout, or to forward the event to an application.

Other information
Note: This applies to any shortcuts collision, e.g.: setting Alt+Shift as keyboard switching shortcut and trying to use Alft+Shift+F as an autoformat shortcut in Intellij Idea.

Reproduce the issue on Cinnamon 3.8.9 with two keyboard layouts, a Ctrl+Shift layout-switch shortcut, and Firefox tab navigation. Trace Cinnamon's keyboard layout shortcut handling and event forwarding; done means longer shortcuts such as Ctrl+Shift+Tab and Alt+Shift+F still reach the application while the layout switch remains usable.