Update conan version & recipes + improvements on GTest & CMake

[piponazo]

In this PR I took actions on several things I had in my "TODO list" for the main branch:

1. Update conan version in CI configurations (from 1.30 to 1.35).
2. Upgrade the conan recipes being used (So that we verify that we can work with latest version of the library dependencies).
3. Remove gtestwrapper.h and use gtest.h directly. That wrapper was used in the past to avoid some compiler warnings/errors, but it is not needed anymore.
4. Use the exiv2lib_int OBJECT library for the unit tests target. This avoids the recompilation of many sources when enabling the unit tests in the project.

Furthermore I had to do other changes to deal with different issues after doing the previous changes:

1. Usage of "-Wno-deprecated-declarations" in travis-ci configuration. The jobs using the clang compiler were finding such warnings when compiling the unit tests. At somepoint we should replace the deprecated functions by the new ones, but that would affect people in older distributions still using GTest 1.8.
2. Travis-CI macOS jobs upgraded to xcode12.2.
3. Now we use the conan cmake_paths generator instead of the cmake one. For our case, it is easier to use, less code, and it also solved an issue I had with macOS (https://travis-ci.org/github/Exiv2/exiv2/jobs/768071045).
4. After all these changes, I could cleanup a bit the CMake code (areas related to conan stuff)

[piponazo]

Long to explain, but 'expat/2.3.0' does not work on windows. So I reverted this change and we keep using 2.2.7