fix: move shiftmode and altmode to global

now shiftmode and altmode is recoreded in Keys module.  So all the UIs
are sharing these two mode instead of remembering their own. These fix
the bug in UI switchings.

For instance, you use combo 'Alt'+'b' to fire up some menu in pdfreader,
the altmode is set, but the alt key was released after the menu show
up. Then the key release event will only be captured by the menu. So
after you exit from the menu, the altmode is still set in pdfreader,
which is not what we want.

inputbox.lua

@@ -102,9 +102,9 @@ function InputBox:input(ypos, height, title, d_text)
 			ev.code = adjustFWKey(ev.code)
 			--local secs, usecs = util.gettime()
 			if ev.code == KEY_SHIFT then
-				self.shiftmode = true
+				Keys.shiftmode = true
 			elseif ev.code == KEY_ALT then
-				self.altmode = true
+				Keys.altmode = true
 			elseif ev.code == KEY_FW_UP then
 			elseif ev.code == KEY_FW_DOWN then
 			elseif ev.code == KEY_A then
@@ -200,10 +200,10 @@ function InputBox:input(ypos, height, title, d_text)
 			--print("E: T="..ev.type.." V="..ev.value.." C="..ev.code.." DUR="..dur)
 		elseif ev.type == EV_KEY and ev.value == EVENT_VALUE_KEY_RELEASE
 		and ev.code == KEY_SHIFT then
-			self.shiftmode = false
+			Keys.shiftmode = false
 		elseif ev.type == EV_KEY and ev.value == EVENT_VALUE_KEY_RELEASE
 		and ev.code == KEY_ALT then
-			self.altmode = false
+			Keys.altmode = false
 		end
 	end
 end