Various XFCE4 issues

@yangxuan8282 reported in #1593 (comment 90777380):

the xfce4 issues exist [since] at least two weaks ago, same on x86/armhf/aarch64, the issues is

• when use xfce4-terminal the whole screen keep blinking

• the panel 2 default size is improper, can't display full icons

• sometime can't display default wallpaper, this happen a lot ( like 1/5 or more )

• the plugins always crash today I build pmos for rpi3 again, not got the screen blinking issues, but the cursor is weird, it always jump 3 or 4 letter forward and back, and the plugins not crash for now, but the other two issues is still here

I'm pretty busy right now, if someone else could confirm this and ideally figure out what we need to do to fix this, it would be highly appreciated.