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/languages/en.xml b/gui/theme/common/languages/en.xml
index 4a0ac7f..3769da5 100755
--- a/gui/theme/common/languages/en.xml
+++ b/gui/theme/common/languages/en.xml
@@ -719,5 +719,8 @@
 		<string name="include_kernel_log">Include Kernel Log</string>
 		<string name="sha2_chk">Use SHA2 for hashing</string>
 		<string name="unable_set_boot_slot">Error changing bootloader boot slot to {1}</string>
+		<string name="unmount_sys_install">Unmount System before installing a ZIP</string>
+		<string name="unmount_system">Unmounting System...</string>
+		<string name="unmount_system_err">Failed unmounting System</string>
 	</resources>
 </language>