terminal: do not instantiate `VMSelection` modal if it is not destroyed yet
terminal: do not instantiate `VMSelection` modal if it is not destroyed yet

`addNewTab` method callers are invoked by two different events at the
moment. This patch prevents instantiating modal more than one if those
events fire twice or more.

koding/koding,cihangir/koding,cihangir/koding,koding/koding,szkl/koding,gokmen/koding,jack89129/koding,andrewjcasal/koding,szkl/koding,alex-ionochkin/koding,drewsetski/koding,szkl/koding,gokmen/koding,sinan/koding,mertaytore/koding,jack89129/koding,szkl/koding,rjeczalik/koding,sinan/koding,cihangir/koding,gokmen/koding,kwagdy/koding-1,alex-ionochkin/koding,acbodine/koding,kwagdy/koding-1,drewsetski/koding,andrewjcasal/koding,cihangir/koding,acbodine/koding,andrewjcasal/koding,rjeczalik/koding,acbodine/koding,sinan/koding,alex-ionochkin/koding,rjeczalik/koding,cihangir/koding,jack89129/koding,sinan/koding,acbodine/koding,alex-ionochkin/koding,jack89129/koding,rjeczalik/koding,koding/koding,usirin/koding,acbodine/koding,szkl/koding,koding/koding,koding/koding,acbodine/koding,jack89129/koding,andrewjcasal/koding,rjeczalik/koding,jack89129/koding,mertaytore/koding,sinan/koding,gokmen/koding,koding/koding,jack89129/koding,mertaytore/koding,kwagdy/koding-1,rjeczalik/koding,kwagdy/koding-1,usirin/koding,mertaytore/koding,drewsetski/koding,gokmen/koding,rjeczalik/koding,kwagdy/koding-1,mertaytore/koding,cihangir/koding,andrewjcasal/koding,szkl/koding,usirin/koding,cihangir/koding,alex-ionochkin/koding,usirin/koding,cihangir/koding,szkl/koding,mertaytore/koding,acbodine/koding,alex-ionochkin/koding,koding/koding,usirin/koding,andrewjcasal/koding,jack89129/koding,andrewjcasal/koding,drewsetski/koding,alex-ionochkin/koding,gokmen/koding,sinan/koding,kwagdy/koding-1,sinan/koding,drewsetski/koding,mertaytore/koding,drewsetski/koding,acbodine/koding,rjeczalik/koding,andrewjcasal/koding,gokmen/koding,alex-ionochkin/koding,szkl/koding,usirin/koding,koding/koding,usirin/koding,drewsetski/koding,kwagdy/koding-1,sinan/koding,kwagdy/koding-1,usirin/koding,gokmen/koding,drewsetski/koding,mertaytore/koding