talkatu/talkatu

Added meson option to select if the demo application will be installed or not

Testing Done:
1. Compiled, installed with the default options and verified that the demo application and its manpage weren't installed.

2. Deleted `build` directory, reconfigured the install option added in this MR with `meson configure -Dinstall-demo=true` and then compiled and installed. This time the demo application and its manpage were installed in my system.

Bugs closed: TALKATU-84

Reviewed at https://reviews.imfreedom.org/r/156/
###############################################################################
# Options
###############################################################################
option(
'nls',
type : 'boolean', value : true,
description : 'Install translation files'
)
option(
'doc',
type : 'boolean', value : true, yield : true,
description : 'build documentation with gtk-doc'
)
option(
'help2man',
type : 'boolean', value : true,
description : 'Whether or not to build man pages from --help output'
)
option(
'install-demo',
type : 'boolean', value : true,
description : 'Whether or not to install demo application'
)
option(
'introspection',
type : 'boolean', value : true, yield : true,
description : 'build gobject-introspection support'
)
option(
'tests',
type : 'boolean', value : true,
description : 'run unit tests'
)
option(
'vapi',
type : 'boolean', value : true,
description : 'Whether or not to build vapi files for gplugin'
)