Show info about APK signature

[Surendrajat]

It'll be nice to parse and display apk signature info after it is decompiled. Since this info can be checked multiple times, it'll be a good idea to store it in a json file. An apklab.json file can be useful for storing such info as well as some other project metadata useful for other features (such as using webviews for a good UX).

Tools to parse signature info:

• Jadx: Shipped with APKLab, uses ApkSig, but CLI tool can't parse certificates. See [feature] get signatures info using CLI tool skylot/jadx#1083
• uber-apk-signer: Shipped with APKLab, uses ApkSig, gives sufficient info using -y --verbose
• apksigner: Not shipped, uses ApkSig, can give info about multiple certificates too using --print-certs --verbose

For now, I think uber-apk-signer will do as it's shipped by default, can give signature info with an expiry date, etc. apksigner can be used in future for better support of this feature or to replace uber-apk-signer entirely.

[Surendrajat]

Related to #175.. can share the same WebView for showing signature info.