Add option to automatically unmount system before installing ZIP

This adds the option to automatically unmount the system partition
before installing a ZIP-file.
This option is enabled by default, This is usefull on SAR-devices
where system is mounted as system_root and ZIPs (i.e. Magisk)
try to unmount system instead of system_root and fail to install.

Change-Id: Id94ae658964679e23aa7fa137e07edf74ee073df
diff --git a/gui/theme/common/watch.xml b/gui/theme/common/watch.xml
index f5d3222..05b7bd2 100755
--- a/gui/theme/common/watch.xml
+++ b/gui/theme/common/watch.xml
@@ -3640,6 +3640,9 @@
 					<condition var1="tw_no_sha2" var2="0"/>
 					<data variable="tw_use_sha2"/>
 				</listitem>
+				<listitem name="{@unmount_sys_install=Unmount System before installing a ZIP}">
+					<data variable="tw_unmount_system"/>
+				</listitem>
 			</listbox>
 
 			<action>