IMM: ImmInstallIME should check already existent registry keys

Generally, an IME setup program can write the registry keys and values on registry settings about keyboard layouts (HKLM\SYSTEM\CurrentControlSet\Control\Keyboard Layouts) before ImmInstallIME function is called. For example, a DDK IME sample (fakeimeu) does write the registry settings of keyboard layouts before it calls ImmInstallIME function.

We should check the pre-existent registry keys of keyboard layouts in ImmInstallIME function. Otherwise installation of some IMEs will be inconsistent (the system will collapse).

The target is "trunk/reactos/dll/win32/imm32/imm.c".