Support Shift+PgUp and Shift+PgDn for selection

[waldyrious]

Unmodified PgUp and PgDn already work; the Shift combinations simply do nothing.

[hishamhm]

In most terminals Shift-PageUp and Shift-PageDown are captured to do terminal scrollback.

I can't seem to get Shift-PageUp and Shift-PageDown to be forwarded to Dit in rxvt-unicode. After some research, it seems that Konsole is able to do it:

https://bugs.kde.org/show_bug.cgi?id=267100

...but I haven't tested it myself.

[waldyrious]

I tried to search a bit for how to configure xterm to enable forwarding of the Shift+PgUp and Shift+PgDn keys, and found some stuff. However, I'm afraid I have another problem with my Shift keys which I need to solve first (shift+home/end for instance don't work in my terminal -- it'll hopefully be fixed soon, see #2 (comment)).