Add a (hidden) config option to increase frame rendering timeout

If you see random failures while rendering frames for huge images,
just open kritarc file and add the following option:

frameRenderingTimeout=60000

Where '60000' is the timeout in milliseconds. The default value is
30 seconds and it seems like it is too small for some weird usecases.

CCBUG:438754
CC:kimageshop@kde.org