text-backend: Allow client hiding of input panel

text-backend: Allow client hiding of input panel

Previously, the hide_input_panel and show_input_panel messages for the text
input protocol were limited to specific cases, such as showing the panel on
activation, or making the panel visible after activation. Now, clients are
allowed to toggle the panel visiblity at will as long as they are the currently
active client

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Tested-by: Silvan Jegen <s.jegen@gmail.com>
Reviewed-by: Jan Arne Petersen <janarne@gmail.com>