Ports the dictionary lookup feature to the fcitx5 version

[zonble]

As the implementation on macOS, a user now can look up a phrase:

• by pressing the "?" key when selecting a candidate in the candidate window.
• by pressing the "?" key when marked a range of the composing buffer.

[lukhnos]

After this PR we will have two dependecies which the last public release (2.5.2) didn't have: libicu-dev and libcjson-dev. Previously we only depend on everything that fcitx5 depends (e.g. libfmt-dev). Could this be a problem for potential package maintainers?

For example we would already have to update our Flatpak settings if we were to build against current HEAD. On the other hand, we can also let the Flatpak distribution (or other dist packages) stay at 2.5.2. I'm just wondering what the increased number of deps mean for development velocity (as we want more features) and ease of distribution. Thoughts?

[lukhnos]

Thanks!

[xatier]

I'm on vacation now, will take a look once I have access to my main Linux desktop (~ end of Jan).

[xatier]

Apologies for the delay.

On a typical Arch box, json-c is an indirect dependency of systemd and pkg-config is a dependency of base-devel, Arch users shouldn't see issues with these two packages.